The present invention is directed to a method of forming patterns to a negative photoresist in the manufacture of semiconductor devices or the like, and more particularly, to a method of forming false patterns near a real pattern on a negative photoresist in accordance with which etching is to be carried out.
In recent years, semiconductor devices have become highly integrated, and keeping pace therewith, unit elements in a semiconductor chip provided by advanced technology are extremely miniaturized. Various patterns employed in the manufacture of such chips are accordingly prepared very minutely. As is generally practiced in the industry, a photoresist light sensitive emulsion is utilized in the formation of various impurity diffusion and electrode contact regions in manufacturing semiconductor devices. The photoresist emulsion is coated on the surface of the oxidized silicon slice or wafer, and for example, the slice is then heated to remove solvents. A photographic mask is accurately aligned using a jig, and is placed in contact with the photoresist film which is then partially exposed to ultraviolet light through the mask. Using a photoresist developer, desired patterns are formed in the photoresist film. Using such patterned photoresist film, a silicon oxide or silicon nitride film formed on the surface of the silicon slice is selectively removed to form an impurity diffusion mask or to define contact regions for the semiconductor device.
There are two kinds of photoresist light sensitive emulsions. One is a negative type, wherein the photoresist emulsion has the property of becoming less soluble in its developer wherever it is polymerized by exposure to ultraviolet radiation. Unexposed areas under opaque parts of the mask coated with chrome can be selectively removed with the photoresist developer. The remaining resist is usually hardened through post-baking and acts as a convenient mask through which the oxide or nitride layer can be etched away. The other kind of photoresist emulsion, namely a positive type emulsion, is utilized in exactly the same manner. However, windows are opened wherever the mask is transparent due to the property that resist areas exposed to ultraviolet light become soluble and can be removed by a photoresist developer. Since negative photoresist emulsions can be processed more quickly than positive photoresist emulsions in most cases, the present invention is directed to a negative photoresist film.
In FIG. 1, a mode to carry out the conventional method of forming patterns is illustrated. A photographic mask 1 made of transparent glass 2 has on its surface a pattern provided by an opaque material 3, such as chrome. The mask 1 is placed on a photoresist film 4 formed on an oxide layer 5, which in turn is formed on a semiconductor substrate 6. By irradiating light from above, areas of the film 4 (other than the area beneath the pattern of opaque material film 3) are exposed to light. The area beneath the pattern 3 remains unexposed.
When the unexposed photoresist is removed by a developer which is, for example, a developer mainly comprising xylene, a photoresist film 4 having a pattern as shown in FIG. 2 is formed on the oxide layer 5. By a subsequent etching process, the oxide layer 5 having a desired pattern remains on the semiconductor substrate 6, and steps for diffusion and formation of contacts are carried out thereafter to provide a semiconductor device.
Often times, it becomes necessary to prepare through a photoresist film 4 a very narrow opening or window 3' as shown in FIG. 3. Recent demand for miniaturize integrated circuits often requires that the lateral width as seen in the drawing be less than 3 .mu.m. In such a case, a mask 1 (FIG. 4) having a plate 2 with a pattern 3, which corresponds to the opening 3' of FIG. 3, is generally used in a manner illustrated in FIG. 1. With any conventional negative photoresist, for example, a negative photoresist in the family of isopropyrene, a so-called swelling of the photoresist occurs during its development process because the exposed and polymerized photoresist is essentially soluble in the developer. Ordinarily, this swelling is caused to shrink by the use of a further rinsing solution, for example, butyl acetate, and a desired pattern can be prepared. However, if the width of the pattern is extremely narrow, photoresist at both sides of the formed pattern adheres to each other at the time of development as shown in FIG. 5A. Even if the rinsing solution is used thereafter, the photoresist is not separated as shown in FIG. 5B. As a result, the pattern is obliterated or burr-like parts are formed.
This swelling of the photoresist depends on the thickness of the photoresist and area of the opening. Accordingly, where the width of an opening of the photoresist film to be formed is approximately 3 .mu.m, with the thickness of the photoresist in the order of 8,000 to 10,000 .ANG., the pattern is lost or burr-like parts appear if the opening 3' is isolated from other openings of the photoresist film, as illustrated in FIG. 3, due to the swelling of the photoresist.